1. Field of the Invention
Apparatus and method for preparing and injecting an air stream with dry ice particles or other sublimable particles to be blasted against a surface to be cleaned.
2. Description of Prior Art
Blasting or cleaning a surface with a stream of high-pressure air mixed with sublimable particles is a well-known art. The most used blast media for this purpose is dry ice particles of various sizes. Dry ice is the solid form of carbon dioxide. One of the advantages of using dry ice over other media such as silica sand, glass beads or steel grit is that upon impact the dry ice instantly returns to a gas state leaving no residue to collect or dispose of Dry ice is also much more forgiving on the surfaces it impacts compared to many other media.
In the case of traditional media, both the media and contaminate being removed must be collected and disposed of properly. If the contaminate is a hazardous substance the used media will also have to be treated as a hazardous substance thereby creating more hazardous waste to contended with. Only the contaminant being cleaned off a surface must be disposed of when dry ice blasting is used thus creating less waste then traditional media blast cleaning systems.
Dry ice blasting can and does replace dangerous and environmentally unfriendly cleaning chemicals thereby reducing the exposure of humans and the environment to these chemicals.
Reducing the use of these chemicals also reduces the chance of improper disposal of chemicals into the environment and improves air quality by eliminating the volatile organic chemicals emitted into the air by many cleaning chemicals.
The art includes two generally available types of dry ice blasting systems that use high-pressure air to facilitate the blasting. The two hose system uses two hoses to transport the air and dry ice separately to a ventri suction type blast nozzle where they are mixed. The second type carries the air and dry ice together in one hose to the blast nozzle. The single hose systems, as they are known, use some type of mechanical means to inject or feed dry ice into the air stream at a source of dry ice to be carried to a blast nozzle by one hose.
One advantage of the single hose system is that they generally produce more blasting power then two hose systems of similar size. The single hose systems also have an ergonomical advantage in that the user must manipulate only one hose to facilitate blasting thus significantly reducing the weight he or she must support.
Dry ice is readily available commercially in various forms including block, nugget, and rice. All forms of dry ice can be used for blasting, but block and sometimes nugget types require additional processing to produce dry ice particles of appropriate size for use in blasting. The rice form is the smallest commercially available form of dry ice particles and requires no additional processing as most dry ice blasting machines are designed to use it.
The problem of how to inject dry ice particles into a stream of air is made difficult by the very problematic nature of dry ice. Dry ice chills or freezes most things it comes in contact with or in close proximity to including the vary mechanisms used to act upon it because of its very low temperature (−78° C./−109° F.). This low temperature can lead to condensation or frozen condensation on or inside the equipment and can, through thermal contraction, substantially change the dimensions of critical components. Therefore it is important to avoid mechanically complex designs in this art in order to maintain good reliability.
Dry ice also attracts moisture from the air to its surface were it freezes and degrades its quality. Dry ice particles tend to aggregate into clumps, especially when moisture is present. Once aggregated into clumps it is difficult or impossible to feed through conventional mechanisms of the art.
Manufactures and users of dry ice blasters are all aware of the difficulties of injecting dry ice into a stream of air. The art has many examples of attempts to overcome these difficulties and to improve the art. The art of injecting dry ice particles into a stream of air would appear to be a simple problem to over come. However the fact that there continues to be efforts to overcome the inherent problems of this art indicates there is still room for improvement in the art of dry ice blasting.
The most important problems in the art that need to be overcome are the problems with the inability to consistently feed a metered amount of dry ice particles into a stream of air to create a consistent ratio of dry ice to air in the blast hose at all times. Often the feed of dry ice is intermittent and inconstant in the present art. A second problem that needs to be addressed is the fact that most of the art relies on complicated and therefore relatively expensive mechanisms to overcome the aforementioned problem. The price of equipment keeps this environmentally friendly method of cleaning out of the hands of the individuals and small and medium sized companies who often continue to depend on cleaning methods that are less friendly to the environment then dry ice blasting.
A device that both consistently feeds and accurately meters dry ice into the air stream via an air lock must solve the problem. Several devices are known that try to perform both functions by using moving elements with cavities that are filled with dry ice particles and then attempt to supply the dry ice into the air stream. For example, star wheels, reciprocating plates and rotary disks with cavities move at a given frequency past a dry ice feed station and then move to alien the dry ice filled cavity with the flow of the air stream to discharge it and mix dry ice into the air stream.
U.S. Pat. No. 4,947,592 (1990) and U.S. Pat. No. 5,109,636 (1992) both to Lloyd disclose a star wheel design and U.S. Pat. No. 5,415,584 (1995) and U.S. Pat. No. 5,492,497 (1996) both to Brooke and U.S. Pat. No. 4,744,181 (1988) to Moore disclose reciprocating plate(s) designs. Both design types feed dry ice and meter dry ice into the air stream with similar mechanisms. In these cases the cavities of the wheel and reciprocating plates are fixed in size. The only way to adjust the ratio of dry ice to air is to adjust the frequency at which the cavities are brought into alignment with the air stream. This works only in restricted limits. Problems arise as the ratio of dry ice to air is reduced. To accomplish this, the rotating wheel or the reciprocating plate(s) must be slowed to decrease the discharge frequency. This slowing tends to create an undesirable pulsing of dry ice in the blast hose and at the nozzle when the cavities are not aligned with the airflow stream.
Another problem with the above designs is the fact that as the cavities turn or move slower the greater the chance that the dry ice in them will aggregate into clumps that can not be discharged into the air stream. These clumps often freeze in the cavities thereby increasing the undesirable pulsing of dry ice. If all the cavities become clogged in this way the mechanism will fail to operate. If the mechanism is stopped for a period of time the dry ice that remains in the cavities, that has not yet been discharged into the air stream, may freeze in the cavities and cause pulsing or may freeze the mechanism so that it cannot be restarted.
U.S. Pat. No. 6,346,035 (2002) to Anderson discloses a device that claims to have overcome the above problems, but in reality the devise still faces several of the above problems with an added one that effects the safety of the operator and those in proximity to the operating blaster. This design uses an auger to feed and meter dry ice into a rotating air lock that feeds the dry ice into the air stream. The rotating air lock rotates at a set speed thereby reducing the pulsing effect caused by changing the frequency at which the cavities are discharged in the above-mentioned art. On shut down the auger stops feeding dry ice to the air lock, but the air lock continues to rotate and the stream of high pressure air continues for a set time. This eliminates the possibility of dry ice freezing in the rotor's feed cavities, but more importantly it creates an unsafe condition. Most regulatory agencies require a “dead man” device on abrading blast cleaning equipment that is to promptly stop the flow of air and abrading media when the operator removes his or her bodily input that keeps the machine operating. The continued airflow and diminishing dry ice flow after shut down of this design negates the function of any “dead man” safety device. The auger feed mechanism of this design is also susceptible to being jammed or clogged by aggregated clumps of dry ice as the above-mentioned art.
U.S. Pat. No. 6,346,035 (2002) to Anderson and U.S. Pat. No. 4,947,592 (1990) and U.S. Pat. No. 5,109,636 (1992) both to Lloyd all require the use of two motors to inject dry ice into the air stream going to the cleaning nozzle. By using two motors the designs uses excessive energy that could have been used directly for the cleaning action. Using two motors also makes the machines more complex and thereby reduces reliability.
Known prior art suffers from a number of the following disadvantages:    (a) The ratio of dry ice to pressurized air is a set ratio or is sometimes limited to a narrow adjustment of the ratio. Limited ratios restrict the flexibility to use the art in cleaning applications.    (b) Pulsing of dry ice from the blast nozzle is a negative possibility in several of these designs.    (c) The designs are complex with many intricate moving parts that are specific to each design.    (d) Two of the designs include multiple motors that reduce the energy that can be used for cleaning.    (e) The designs have a relatively high number of parts that are complex in nature. These complex designs increase manufacturing costs by increasing the need for more manufacturing/warehouse space, manufacturing plant equipment, and highly trained employees.    (f) The designs have tendencies to become clogged with aggregated clumps of dry ice.    (g) The designs must keep parts dimensionally within tolerance to avoid the negative effects of the extreme low temperature. The low temperature can change the dimensions of some parts so drastically that thy can no longer function properly.    (h) The designs must continue to purge pressurized air from the nozzle after the actuation trigger is released to remove dry ice from the system thus causing a safety hazard.